Rock drill having a triple coaxial hammer

ABSTRACT

A triple coaxial hammer comprised of three hammer elements in telescopic relation is provided for a rock drill and caused to rapidly reciprocate to pound a drill-steel. The hammer is hydraulically movable on a work stroke, and is returnable by hydraulic force supplemented by force of pressure air. The telescoped arrangement of the hammers provides a compact hammer assembly which effects a relatively wide impact pulse against the drill-steel.

United States Patent [191 Amtsberg Mar. 12, 1974 ROCK DRILL HAVING A TRIPLE COAXIAL HAMMER [76] lnventor: Lester A. Amtsberg, 2200 Bleecher St., Utica, NY. 13503 [22] Filed: Mar. 10, 1972 [21] Appl. No.: 233,592

[52] US. Cl 173/102, 173/57, 173/76, 173/103, 173/126, 173/139 [51] Int. Cl. E21b l/02, E210 7/00 [58] Field of Search 173/102, 103, 126, 128, 173/131, 133, 134, 139; 91/411 R-, 411 C [56] References Cited Y UNITED'STATES PATENTS 682,492 10/1901 Payton 173/103 2,946,314 7/1960 Nast 173/102 1,802,987 4/1931 Shook 173/103 2,800,884 7/1957 Mori 173/103 X FOREIGN PATENTS ORAPPLICATIONS 312,038 3/1930 Great Britain 173/103 Primary ExaminerMarvin A. Champion Assistant Examiner-William F. Pate, III Attorney, Agent, or FirmStephen J. Rudy [57] ABSTRACT A triple coaxial hammer comprised of three hammer elements in telescopic relation is provided for a rock drill and caused to rapidly reciprocate to pound a drill-steel. The hammer is hydraulically movable on a work stroke, and is returnable by hydraulic force supplemented by force of pressure air. The telescoped arrangement of the hammers provides a compact ham- -mer assembly which effects a relatively wide impact pulse against the drill-steel.

18 Claims, 4 Drawing Figures ROCK DRILL HAVING A TRIPLE COAXIAL HAMMER BACKGROUND OF THE INVENTION This invention relates to a percussive rock drill having a drill-steel arranged to be driven into the work under repeated impacts delivered by a reciprocating hammer. It is especially concerned with a rock drill having an improved hammer which comprises multiple hammer elements in telescoped arrangement.

It is known in the drilling of rock with a percussive tool that the efficiency of drilling is a function of the capacity of the rock to absorb the impact energy with a minimum of reflection; and it is known that such capacity can be enhanced by providing stress waves of optimum duration. This is frequently referred to as optimum pulse width when referring to the shape of the stress wave in the drill-steel displayed on an oscilliscope.

An optimum pulse width is three to eight times greater than that produced by a conventional pistonhammer striking a conventional drill-steel; and it varies with stiffness of the rock. The pulse can be widened by introducing a flexible member between the hammer and the drill-steel or by making the hammer itself a flexible member, as indicated in US. Pat. No. 3,570,609.

It is also known that the pulse width is determined primarily by the length of the hammer if no supplementary flexible member is included, as indicated in US Pat. No. 1,559,709. A fault of this principle is in its application, since the length of hammer required for a reasonable match with average rock would result in excessive length of machine.

The objective of this invention is to provide an adequate pulse width by folding the hammer in telescoped or nested fashion to obtain a substantial sonic length and yet have a reasonable dimensional length; that is, without unduly increasing the physical length of the hammer.

This objective is obtained in the present invention by providing a triple coaxial hammer; that is, a hammer assembly comprising three individual hammer elements disposed one within the other in telescoped or nested fashion. By means of this arrangement a compressive wave originating at the impact end of the outer hammer element will travel to the opposite end where it will create a tension wave in the intermediate hammer element which in turn will create a compression wave in the inner hammer element to produce a desirable pulse width upon the anvil end of a drill-steel. Endwise contact between the three hammer elements is maintained by an acceleration force produced by a power source, such as a piston, the mass of which can be very small. The piston can be a separate member that pushes against the inner hammer element; or it can be a head made integral with or fixed to a free end of the inner hammer element. The cross sectional area of each hammer element is desirably nearly equal to that of the drill steel rod.

In the hammer assembly of the present invention, the three hammer elements are disposed in loose sliding relation to one another; that is, they are not fixed or fastened to one another. In such an arrangement, it is obvious that the force required for the return stroke of the assembly must be applied to the outer hammer element. Here, it is done by axially directed forces without adding to the assembly any lumped mass of significant value, thus undesirable stress peak is avoided.

The sonic length of the triple coaxial hammer is determinable by the sum of the effective length of each hammer element. A formula as set forth below may be used for this purpose:

LS (micro-seconds) L, L, L (feet) X IO /Speed of sound in the hammer material (ft/sec.)

When the cross-sectional area of each hammer element is nearly equal to that of the drill steel, the pulse width (duration) will be equal to approximately twice the sonic length of the hammer assembly.

BRIEF DESCRIPTION OF DRAWING In the accompanying drawing:

FIG. 1 is a sectional view of a telescoped hammer assembly embodying the invention;

FIG. 2 is a view in section illustrating an hydraulic rock drill in which the telescoped hammer assembly is incorporated; the hammer assembly is shown in its returned condition preparatory to a downstroke;

FIG. 3 is a detail of FIG. 2 showing the position of the hammer assembly and valve at the time of impact with the drill-steel; and

FIG. 4 is a detail of FIG. 2 showing the valve moved by the returning hammer assembly to a position blocking both the inlet and return annuli just prior to the FIG. 2 position being obtained; and further showing the moved position of the pressure relief plug.

DESCRIPTION OF PREFERRED EMBODIMENT Reference is now directed to FIG. 1 wherein is disclosed for use in a percussive type rock drill a telescoped hammer assembly 10 which is axially aligned with a conventional impact receiving anvil rod or drillsteel 11. The hammer assembly is reciprocable relative to the drill-steel to pound the latter against the work such as a rock surface. The hammer assembly has a retracted or returned position, as in FIG. 1, wherein its bottom or impacting end is spaced as at 12 from the drill-steel. It is adapted under pressure of hydraulic oil applied to its rear or upper end to be forcefully driven into impacting relation with the drill-steel.

The hammer assembly comprises a plurality of hammer members (here, three) namely, an outer hammer member 14, an inner hammer member 15 and an intermediate hammer member 16. The three members are loosely telescoped or nested, one within the other. The outer hammer 14 has a cylindrical tubular body which is open at its rear and is closed by means of a head 17 at its bottom or impacting end. The intermediate hammer 16 has a cylindrical tubular body which is open at its rear and is closed by a head 18 at its bottom. The intermediate hammer has a radial flange 19 around its rear which overlies or abuts the corresponding end of the outer hammer so as to maintain the head 18 of the intermediate hammer spaced from and in unseated relation, as at 21, to the inside surface of the head 17 of the outer hammer. The inner hammer 15 is a heavy solid cylindrical rod having a bottom end which seats upon the inside surface of the head 18 of the intermediate member; and it has a rear end portion which projects axially beyond the intermediate and outer hammers.

Axially aligned with, and seated upon a flat end of the inner hammer is a pusher piston 22. The latter may be a head integral with, or fixed to, the inner hammer; or, as here, it may be a separate element. It is desired that the piston be as light as possible in weight so that it does not act as a lumped mass that would create an undesirable stress peak in the hammer and drill-steel systems. Its purpose is to forcefully push, not to pound, the hammer assembly.

Hydraulic oil pressure applied to the piston 22 acts through the inner hammer 15 to push the entire hammer assembly downward into forceful impacting relation with the drill-steel. As the outer hammer 14 impacts against the drill-steel, a compressive or shock wave originating at the impacting end of the outer hammer travels through its body to its rear end to create a wave of tension in the intermediate hammer 16 which in turn develops a wave of compression in the inner hammer 15. In effect, as the outer hammer makes impact with the drill-steel, the resultant shock wave exerts a rearward force upon the flange 19 of the intermediate member, but since the intermediate member is at this time in effect still moving forwardly, the pulse of the outer hammer 14 against the drill-steel is continued; and this pulse is further prolonged as the heavy inner hammer l continues in effect to move forwardly in time duration beyond that of the intermediate hammer 16. This overall action of the hammer assembly widens the impact pulse or prolongs, it against the drill-steel and thus effects a prolonged penetrating action of the drill-steel against the work rock. The bounce and the wasted energy usually accompanying bouncing that would otherwise occur on impact of a conventional hammer against a drill-steel is in effect avoided by the hammer assembly of the present invention.

Return forces caused to be automatically applied to the outer hammer immediately following the overall impacting action serve to return the hammer assembly to its retracted position. For this purpose, pressure air may be applied to the bottom of the hammer assembly and an hydraulically actuable sleeve piston 45 may be engaged against a shoulder 63 of the outer hammer to carry the assembly in a return direction. While the hammer elements are separate from one another, endwise contact between the individual hammers is maintained at all times by accelerating forces applied to the outer hammer in the return direction and by the force of the piston 22 acting through the inner hammer in the work direction.

As earlier explained, the pulse width (time duration) obtainable varies with the sonic length of the hammer assembly and with the cross-sectional area of the individual hamriier elements relative to that of the drillsteel. By making, as here, the cross sectional area of each of the individual hammer elements nearly equal to that of the drill-steel, a desirable pulse width is obtainable which is approximately equal to twice the sonic length of the hammer assembly.

The hammer assembly is shown in FIGS. 2-4 incorporated in an hydraulically operable rock drill 28 of a drifter type for impacting the end of the anvil end 29 of a drillsteel, generally designated 11, under pressure of hydraulic oil applied to a pusher piston 22. The anvil portion 29 of the drill-steel is adapted by threading 31 to be coupled to extensions, as needed, of drill rod.

The drill steel 11 is adapted to be rotated during drilling operations by a suitable motor (not shown). Rotation of the motor is 'transmitted to the drill-steel through a gear 32 having a driving end connection at 33 with a chuck 34 which in turn is internally splined to a splined portion 35 of the drill-steel.

The drill-steel is axially aligned with the hammer assembly; and it has a flat end for receivingimpacts from a corresponding flat end of the outer member 14 of the hammer assembly. When pressed against the work rock, the drill-steel is slidably retractible into the housing of the tool to a position, as in FIG. 2, for receiving the impacts of the hammer assembly. An annular end shoulder of a bushing 36 fixed to gear 32 is cooperable with end shoulders at 37 of the splined portion 35 of the drill-steel to limit the extent to which the drill-steel may be retracted. A retainer, defined by a sleeve piston 38 having limited axial movement in a chamber 39 of an end cap 41 of the housing, is cooperable with splined end shoulders at 42 of the drill-steel to curb outward axial movement of the latter.

The retaining piston 38, being under continuous pressure of hydraulic oil from a branch supply line 43, is constantly urged upwardly in its chamber 39. Further, when the drill-steel is being withdrawn with the tool out of pressed relation to the work, the hydraulically cushioned retaining piston 38 cooperates with the shoulder 42 of the drill-steel to absorb the energy of any undesirable hammer impacting that might continue to be transmitted to the drill-steel.

Pressure air that is constantly feeding into a small chamber 44 into which the anvil end 29 of the drillsteel projects, enters and flows down a bore 40 of the drill-steel to clear away cuttings at the bottom of the drill hole. The lower end of the hammer assembly 10 is slidably located above the drill-steel in chamber 44. The pressure of air entering chamber 44 exerts a biasing thrust upon the hammer assembly to provide some of the force used in returning the assembly to its return position following an impacting action.

The major force for returning the hammer assembly is applied by a return sleeve piston 45. The latter provides an internal bearing surface about the rear portion of the hammer assembly. The return piston is confined in a stationary block 46 of the housing for relative limited axial movement. A bushing 47 fitted in a lower portion of the block provides a further bearing surface for the hammer assembly. A stop 49 provided by an overhanging end wall portion of a cylinder 51 surmounting the block 46 limits the extent of rearward movement of the return piston. The pusher piston 22, together with a rear portion of the hammer assembly, projects rearwardly beyond the return piston 45 into the cylinder 51. A chamber area 50 of the cylinder below the pusher piston 22 is at all times vented through ports 52 to an annulus 53 connected with an hydraulic return line 54 so as not to inhibit rearward movement of the return piston 45 and hammer assembly.

An annular chamber 58 below the'return piston 45 is connected at all times through a port 59 with a pressurized hydraulic oil reservoir 61 whereby the return piston is constantly biased in the direction of the stop 49.

During a work or downstroke of the hammer assembly, a shoulder 63 about the rear end of the outer hammer is cooperable with a complementary shoulder of the return piston 45 to move the latter downwardly against the opposing bias of hydraulic fluid in chamber 58. A shuttle valve 68 is caused during this downstroke to be shifted so as to block off hydraulic operating fluid through annulus 75 to chamber 67 and so as to relieve chamber 67 of the pressure of hydraulic fluid therein through annulus 76 to the return line 54. This event enables thrust of pressure of hydraulic fluid from passage 59 to chamber 58 on the return piston 45 to sharply accelerate the hammer assembly in a return direction immediately following impacting action of the hammer assembly with the drill-steel 29.

The travel of the return piston 45 is brief, being lim- 'ited by the closely spaced overhanging stop 49, so that the return piston acts upon the hammer assembly for only a fraction of the return stroke of the hammer assembly during which time it provides rapid acceleration of the assembly to a desired return speed. This speed is then maintained for the remainder of the return stroke by the moderate force of the pressure air acting in chamber 44 on the bottom of the assembly. This feature is of decided advantage in that it provides a maximum average return speed of the assembly with a minimum of final speed and, therefore, develops a minimum of return stroke kinetic energy.

The hydraulic reservoir 61 is defined by an outer jacket 64 of the housing. It is connected by means of a main supply passage 65 with an external pressure oil supply system (not shown) whereby the reservoir fluid is maintained at a constant predetermined pressure.

The shuttle valve 68 is operable in chamber 67 rearwardly of the pusher piston 22. It controls application to, and relief of, pressurized reservoir oil from the pusher piston. The valve is defined by means of an annular body 69 joined by means of a group of internal spokes 71 (FIG. 4) to an axially extending valve drive rod 72.

The body of the valve is slidable between a pair of axially spaced annular seats or shoulders 73 and 74 in chamber 67 (FIG. 3) relative to an inlet annulus 75 and to a return annulus 76. The inlet annulus is connected through a ring of ports with the reservoir 61; the return annulus is connected through a ring of ports with the return line 54. The latter empties into a sump portion of the external pressure oil supply system.

The valve rod 72 is axially aligned with the pusher piston 22. It has a rear portion 79 slidable in a bushing 81. The rear end of the rod is exposed to a chamber 82 vented at all times by a passage 83 to the return line 54. The valve rod 72 has an opposite tubular end 84 which is slidable in a bushing 85 (FIG. 3) provided in a closure head of the pusher piston 22 and is exposed to pressures developing in aninternal chamber 86 of the pusher piston.

An annular rubber pad 87 (FIG. 4) in the head of the pusher piston is cooperable toward the end of a return stroke of the hammer assembly with the spokes 71 of the valve to engage and shift the valve to a partially open condition relative to the inlet annulus 75 and to a closed condition relative to the return annulus 76. This position of the valve is not shown but occurs shortly after the position obtained in FIG. 4.

This initial partial shifting of the valve allows flow of reservoir fluid through annulus 75 and through ports 88 to the internal piston chamber 86 and to chamber 67 rearwardly of the head of the pusher piston to arrest the return movement of the hammer assembly. Fluid pressure then developing in the tubular end 84 of the valve rod serves to shift the valve 68 further rearwardly to its fully open condition relative. to the inlet annulus 75, as in FIG. 2, and to hold it there until pressure builds up about the pusher piston sufficiently to forcefully push the hammer assembly downward on an impact stroke against the drill-steel.

At about the time the hammer assembly next reaches midway of its downstroke, chamber 86 is blocked from chamber 67; and the side ports 88 in the pusher piston communicate with relief ports 89 to allow relief to the return line 54 of fluid from the internal piston chamber 86 and consequent relaxation of pressure in the tubular end 84 of the valve rod. A differential fluid pressure developing about the upper portion of the valve rod at this time forces the valve 68 down to its reverse or closed position (as in FIG. 3) relative to the inlet annulus 75, and to an open position relative to the return annulus 76 preparatory to a return stroke of the hammer assembly.

During the return stroke of the hammer assembly, the pressure fluid about the pusher piston is forced from the internal chamber 86 through the side ports 88 and from chamber 67 above the head of the pusher piston to the return line 54. To avoid excessive peaks of energy in the return passages which might inhibit return speed of the hammer assembly and decrease its operating frequency, the return line is fitted with a low pressure accumulator 93 which tends to avoid development of excessive peaks of energy in the return line. The accumulator has a large diameter connection 93 with the return line. It is fitted with a cup-piston 94 and is pressurized through a check valve 95 rearwardly of the cup-piston with air or an inert gas to 15 to 25 psi.

To minimize leakage from the chamber 67 rearwardly of the pusher piston 22, the body 69 of the shuttle valve is slightly wider than the distance between the inlet and return annuli 75, 76. This has the effect of blocking all exits from chamber 67 for a brief period during the latter portion of the return stroke of the pusher piston when the piston is pushing the shuttle valve through its shift point, as shown in FIG; 4. To avoid excessive pressure build-up of hydraulic fluid in chamber 67 at this time, chamber 67 is fitted with a loose fitting plug 96 in a side passage 97 communicating with the reservoir 61. The plug in being moved under this pressure build-up against opposing pressure of the reservoir, will limit the pressure in chamber 67 to that of the reservoir. A cross-pin 98 prevents the plug from being blown into the reservoir. The side passage 97 is sufficiently adequate to accommodate the fluid displaced by the returning pusher piston during the brief period when the valve blocks both the inlet and return annuli. The plug will'return to its seat 99 in the passage during'the latter part of the downstroke of the driving piston when oil flow from chamber 67 to the return line 54 is sufficient to cause a small pressure differential between the reservoir 61 and chamber 67 pressures;

Summarizing the operation of the rock drill, assuming the tool has obtained the returned condition of the hammer assembly as shown in FIG. 2, reservoir oil pressure then entering and building up in chambers 67 and 86 about the pusher piston 22 acts through the latter to forcefully push the hammer assembly 10 downward into impacting relation with the drill-steel.

At about midway of the downstroke of the hammer assembly, chamber 86 is blocked off from chamber 67 and pressure oil in chamber 86 is relieved through ports 88, 89 to the return line 54. A differential area pressure then developing over the upper portion of the valve rod shifts the valve down to its reverse position (FIG. 3) where it closes chamber 67 to the inlet annulus 75 and opens chamber 67 to the return annulus 76. Momentarily before impacting is effected, the shoulder 63 of the outer hammer of the hammer assembly engages the return piston 45 and forces the latter downwardly against the bias of reservoir fluid pressure in chamber 58.

As the outer hammer 14 next impacts against the drill-steel, the momentum of the intermediate and inner hammers 16, 15 continues in order forwardly to prolong the impact pulse. No undesirable bounce .with its consequent wasted energy occurs in the hammer assembly with the impact. The continued forward momentum of the inner hammer 15 avoids this. Upon substantial completion of the prolonged pulse and following shifting of the valve to its reverse position, fluid pressure over the pusher piston will have been substantially relaxed from chambers 86 and 67 to the return line 54. The air pressure constantly feeding into the chamber 44 below the hammer assembly, and the hydraulic bias in chamber 58 acting on the return piston 45 then operate to rapidly return the hammer assembly rearwardly. The reciprocating stroke of the hammer assembly is relatively short as indicated by the space below the assembly in FIG. 2. The return thrust of the return piston is brief as it limits against the stop 49. However, the force imparted by'it to the hammer assembly, together with the moderate force of the air pressure in chamber 44, serve to continue the return movement of the hammer assembly. As the hammer assembly returns, any residual fluid trapped in the internal piston chamber 86 and that in chamber 67 above the head of the pusher piston is relieved through the various exits to the return line 54. Undesirable excess pressures that might inhibit return of the assembly are preventedfrom developing in the return line'at this time by the accumulator 92.

Near the end of the return stroke of the hammer assembly, the body 69 of the pusher piston closes over the relief annulus 89; and the piston pad 87 engages the spokes 71 of the valve to push the valve to a closed condition over the return annulus 76 and to a partially open condition over the inlet annulus 75. Momentarily before the inlet annulus is partially opened, the valve obtains, as in FIG. 4, a position closed over both the inlet and return annuli, as earlier explained. At this time, any undesirable fluid pressure that might develop in chamber 67 to impede further return of the piston assembly is dissipated by lateral movement of the plug 96 in the side passage 97 against the pressure of fluid in the reservoir 61.

As the valve then partially opens the inlet annulus 76, reservoir pressure fluid enters chambers 67 and 86 to arrest further return movement of the hammer assembly. The pressure developing in chamber 86 then acts on the tubular end 84 of the valve rod to shift the valve to its fully open condition, as in FIG. 2, relative to the inlet annulus 75. Fluid pressure then rapidly developing about the pusher piston 22 again forcefully pushes the hammer assembly downward on its work stroke, as before described.

lt is to be noted that it is not essential that the inner hammer member 15 be solid. Nor is it essential that the inner hammer member project beyond the other hammer members. In lieu of the projection, a stern may be attached to the pusher piston so as to slidably depend part way into the intermediate hammer into contact relation with the inner hammer.

It is also understood that the hammer members may be arranged in a reverse condition to that shown in FIG. 1, so that the inner member effects impacting with the drill steel and the outer member is utilized for returning the hammer assembly.

I claim:

1. A rock drill comprising a housing; a drill-steel retractible into the housing; a hammer unit-aligned with the drill steel for imparting impacts to the latter; shoulder means in the housing cooperable with shoulder means on the drill-steel for limiting the retracted position of the latter; a radial shoulder about the outer surface of the hammer unit; a returnpiston about the hammer unit having cooperation with the radial shoulder for exerting a force upon the hammer in a return direction away from the drill-steel; means hydraulically biasing the return piston in a return direction'of the hammer; and stop means having cooperation with an end of the return piston for determining the duration of cooperation of the return piston with the radial shoulder of the hammer in a return direction.

2. A rock drill as in claim 1, wherein means is provided for exerting a force supplemental to that of the return piston on the hammer unit in a return direction and for continuing said force after cooperation of the return piston with the stop means.

3. A rock drill asin claim 1, wherein the hammer unit comprises multiple individual hammers in nested relation to one another.

4. A rock drill as in claim 3, wherein an hydraulically actuable driving means is in axial engagement with a rear end of the innermost hammer.

5. A rock drill as in claim 1, wherein the hammer unit comprises three individual hammers loosely disposed one within the other, the outer one of which is tubular with a bottom end defining an impact transmitting head, the intermediate one of which is tubular having a bottom. head spaced above the inner bottom surface of the outer hammer and having a flange at its opposite end abutting an end wall of the outer hammer; and the innermost one of which is seated within the intermediate hammer and has a driving force receiving end.

6. A rock drill as in claim 5 wherein an hydraulically actuable pusher piston is axially arranged upon the driving force receiving end of the innermost hammer for imparting a driving force to the latter.

7. A rock drill as in claim 6, wherein the hammer unit is slidably supported in a housing, the pusher piston is operable in a cylinder of the housing and has a head exposed to a chamber of the cylinder, and valve means is operable in the chamber rearwardly of the pusher piston for controlling application of hydraulic driving fluid to and relief of such fluid from the chamber.

8. A rock drill as in claim 7, wherein an inlet annulus communicates the chamber radially with a pressurized hydraulic supply system, a return annulus spaced axially from the inlet annulus connects the chamber radially with a return line to the supply system, and the valve has a cylindrical body shiftable from a first position closed over the inlet annulus and opened over the return annulus to a second position having a reverse condition.

9. A rock drill as in claim 8, wherein the body of the valve has an axial dimension which is sufficient in an intermediate shifted position of the valve to close over both the inlet and return annuli.

10. A rock drill as in claim 8, wherein the valve has a spoked internal web whereby the head of the pusher piston is at all times exposed to areas of the chamber at opposite surface areas of the web.

11. A rock drill as in claim 10, wherein means is provided for relieving excessive fluid pressure that might develop in the chamber above the head of the pusher piston when the valve is closed over both the inlet and return annuli.

12. A rock drill as in claim 10, wherein said means for relieving the excessive pressure is a plug loosely fitting in a passage connecting the chamber with the hydraulic supply system.

13. A triple coaxial hammer assembly for transmitting impacts to a drill-steel, comprising an outer tubular hammer member having an open upper end and a closed bottom end defining a head for transmitting impacts directly to the drill-steel; an intermediate tubular hammer member received in the open end of the outer hammer member having a peripheral flanged upper end lying in unattached abutment with the upper end of the outer hammer member and having a closed bottom end disposed in spaced relation to the inner side of the bottom of the outer hammer member; an inner hammer member received in the intermediate hammer member having a bottom end seated upon the inner side of the bottom of the intermediate hammer member; and means for pushing the hammer assembly to bring the head of the outer hammer member into impacting relation with the drill-steel.

14. A triple coaxial hammer assembly as in claim 13,

wherein the means for pushing the hammer assembly is integral with the inner hammer member.

15. A triple coaxial hammer assembly as in claim 13, wherein the cross-sectional area of each of the hammer members differs slightly from one another.

16. The combination of a triple coaxial hammer assembly and a drill-steel normally spaced from the assembly for receiving impacts from the latter, wherein the assembly comprises three unattached hammer members in coaxial telescoped relation to one another and axially aligned with the drill-steel, the outer member of which assembly has a head for transmitting impacts to the drill-steel, the intermediate member of which limits by means of a flange at an upper end upon an end wall of the outer member and has a head disposed in unseated relation to the inner bottom surface of the outer member; and the inner member of which is substantially solid, has a bottom end seated on the inner bottom surface of the intermediate member and has an opposite end projecting axially beyond the other two members; and piston means is disposed in seated axial relation to the inner member for transmitting a pushing force through the inner and intermediate members to the outer member.

17. The combination as in claim 8, wherein the crosssectional area of each hammer member is substantially equal to that of the drill-steel.

18. A percussion tool having a hammer comprised of a number of telescoped separate cylindrical members with endwise abutting shoulders arranged for sequential transmission of axial force waves means holding, said members in abutting contact by applying downstroke force to the inner member and return stroke force to the outer member. 

1. A rock drill comprising a housing; a drill-steel retractible into the housing; a hammer unit aligned with the drill steel for imparting impacts to the latter; shoulder means in the housing cooperable with shoulder means on the drill-steel for limiting the retracted position of the latter; a radial shoulder about the outer surface of the hammer unit; a return piston about the hammer unit having cooperation with the radial shoulder for exerting a force upon the hammer in a return direction away from the drill-steel; means hydraulically biasing the return piston in a return direction of the hammer; and stop means having cooperation with an end of the return piston for determining the duration of cooperation of the return piston with the radial shoulder of the hammer in a return direction.
 2. A rock drill as in claim 1, wherein means is provided for exerting a force supplemental to that of the return piston on the hammer unit in a return direction and for continuing said force after cooperation of the return piston with the stop means.
 3. A rock drill as in claim 1, wherein the hammer unit comprises multiple individual hammers in nested relation to one another.
 4. A rock drill as in claim 3, wherein an hydraulically actuable driving means is in axial engagement with a rear end of the innermost hammer.
 5. A rock drill as in claim 1, wherein the hammer unit comprises three individual hammers loosely disposed one within the other, the outer one of which is tubular with a bottom end defining an impact transmitting head, the intermediate one of which is tubular having a bottom head spaced above the inner bottom surface of the outer hammer and having a flange at its opposite end abutting an end wall of the outer hammer; and the innermost one of which is seated within the intermediate hammer and has a driving force receiving end.
 6. A rock drill as in claim 5, wherein an hydraulically actuable pusher piston is axially arranged upon the driving force receiving end of the innermost hammer for imparting a driving force to the latter.
 7. A rock drill as in claim 6, wherein the hammer unit is slidably supported in a housing, the pusher piston is operable in a cylinder of the housing and has a head exposed to a chamber of the cylinder, and valve means is operable in the chamber rearwardly of the pusher piston for controlling application of hydraulic driving fluid to and relief of such fluid from the chamber.
 8. A rock drill as in claim 7, wherein an inlet annulus communicates the chamber radially with a pressurized hydraulic supply system, a return annulus spaced axially from the inlet annulus connects the chamber radially with a return line to the supply system, and the valve has a cylindrical body shiftable from a first position closed over the inlet annulus and opened over the return annulus to a second position having a reverse condition.
 9. A rock drill as in claim 8, wherein the body of the valve has an axial dimension which is sufficient in an intermediate shifted position of the valve to close over both the inlet and return annuli.
 10. A rock drill as in claim 8, wherein the valve has a spoked internal web whereby the head of the pusher piston is at all times exposed to areas of the chamber at opposite surface areas of the web.
 11. A rock drill as in claim 10, wherein means is provided for relieving excessive fluid pressure that might develop in the chamber above the head of the pusher piston when the valve is closed over both the inlet and return annuli.
 12. A rock drill as in claim 10, wherein said means for relieving the excessive pressure is a plug loosely fitting in a passage connecting the chamber with the hydraulic supply system.
 13. A triple coaxial hammer assembly for transmitting impacts to a drill-steel, comprising an outer tubular hammer member having an open upper end and a closed bottom end defining a head for transmitting impacts directly to the drill-steel; an intermediate tubular hammer member received in the open end of the outer hammer member having a peripheral flanged upper end lying in unattached abutment with the upper end of the outer hammer member and having a closed bottom end disposed in spaced relation to the inner side of the bottom of the outer hammer member; an inner hammer member received in the intermediate hammer member having a bottom end seated upon the inner side of the bottom of the intermediate hammer member; and means for pushing the hammer assembly to bring the head of the outer hammer member into impacting relation with the drill-steel.
 14. A triple coaxial hammer assembly as in claim 13, wherein the means for pushing the hammer assembly is integral with the inner hammer member.
 15. A triple coaxial hammer assembly as in claim 13, wherein the cross-sectional area of each of the hammer members differs slightly from one another.
 16. The combination of a triple coaxial hammer assembly and a drill-steel normally spaced from the assembly for receiving impacts from the latter, wherein the assembly comprises three unattached hammer members in coaxial telescoped relation to one another and axially aligned with the drill-steel, the outer member of which assembly has a head for transmitting impacts to the drill-steel, the intermediate member of which limits by means of a flange at an upper end upon an end wall of the outer member and has a head disposed in unseated relation to the inner bottom surface of the outer member; and the inner member of which is substantially solid, has a bottom end seated on the inner bottom surface of the intermediate member and has an opposite end projecting axially beyond the other two members; and piston means is disposed in seated axial relation to the inner member for transmitting a pushing force through the inner and intermediate members to the outer member.
 17. The combination as in claim 8, wherein the cross-sectional area of each hammer member is substantially equal to that of the drill-steel.
 18. A percussion tool having a hammer comprised of a number of telescoped separate cylindrical members with endwise abutting shoulders arranged for sequential transmission of axial force waves means holding, said members in abutting contact by applying downstroke force to the inner member and return stroke force to the outer member. 